Method and apparatus for processing video signal

ABSTRACT

A method for decoding a video according to the present invention may comprise: determining an intra prediction mode of a current block, deriving reference samples from neighboring samples of the current block, obtaining a first prediction sample for the current block, based on the intra prediction mode and the reference samples, determining an offset for the first prediction sample, and obtaining a second prediction sample by applying the offset to the first prediction sample.

TECHNICAL FIELD

The present invention relates to a method and an apparatus for processing video signal.

BACKGROUND ART

Recently, demands for high-resolution and high-quality images such as high definition (HD) images and ultra-high definition (UHD) images have increased in various application fields. However, higher resolution and quality image data has increasing amounts of data in comparison with conventional image data. Therefore, when transmitting image data by using a medium such as conventional wired and wireless broadband networks, or when storing image data by using a conventional storage medium, costs of transmitting and storing increase. In order to solve these problems occurring with an increase in resolution and quality of image data, high-efficiency image encoding/decoding techniques may be utilized.

Image compression technology includes various techniques, including: an inter-prediction technique of predicting a pixel value included in a current picture from a previous or subsequent picture of the current picture; an intra-prediction technique of predicting a pixel value included in a current picture by using pixel information in the current picture; an entropy encoding technique of assigning a short code to a value with a high appearance frequency and assigning a long code to a value with a low appearance frequency; etc. Image data may be effectively compressed by using such image compression technology, and may be transmitted or stored.

In the meantime, with demands for high-resolution images, demands for stereographic image content, which is a new image service, have also increased. A video compression technique for effectively providing stereographic image content with high resolution and ultra-high resolution is being discussed.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method and an apparatus for efficiently performing intra-prediction for an encoding/decoding target block in encoding/decoding a video signal.

An object of the present invention is to provide a method and an apparatus for performing intra prediction through a weighted prediction using a plurality of reference samples in encoding/decoding a video signal.

An object of the present invention is to provide a method and an apparatus for refining a prediction sample generated through intra prediction using an offset in encoding/decoding a video signal.

An object of the present invention is to provide a method and an apparatus for refining a prediction sample generated through intra prediction using an offset, and further using a different offset in a predetermined unit in encoding/decoding a video signal.

The technical objects to be achieved by the present invention are not limited to the above-mentioned technical problems. And, other technical problems that are not mentioned will be apparently understood to those skilled in the art from the following description.

Technical Solution

A method and an apparatus for decoding a video signal according to the present invention may determine an intra prediction mode of a current block, derive reference samples from neighboring samples of the current block, obtain a first prediction sample for the current block, based on the intra prediction mode and the reference samples, determine an offset for the first prediction sample, and obtain a second prediction sample by applying the offset to the first prediction sample.

A method and an apparatus for encoding a video signal according to the present invention may determine an intra prediction mode of a current block, derive reference samples from neighboring samples of the current block, obtain a first prediction sample for the current block, based on the intra prediction mode and the reference samples, determine an offset for the first prediction sample, and obtain a second prediction sample by applying the offset to the first prediction sample.

In a method and an apparatus for encoding/decoding a video signal according to the present invention, whether to apply the offset to the first prediction sample may be determined based on the intra prediction mode of the current block.

In a method and an apparatus for encoding/decoding a video signal according to the present invention, the offset may be determined based on a weighted sum of the reference samples.

In a method and an apparatus for encoding/decoding a video signal according to the present invention, a weight applied to each of the reference samples is determined based on a distance from the first prediction sample.

In a method and an apparatus for encoding/decoding a video signal according to the present invention, the reference samples include a reference sample at a fixed position and a reference sample determined dependently on a position of the first prediction samples.

In a method and an apparatus for encoding/decoding a video signal according to the present invention, the reference sample at the fixed position may include a reference sample adjacent to a top left corner of the current block, and the reference sample determined dependently on the position of the first prediction sample may include at least one of a reference sample on a same horizontal line as the first prediction sample or a reference sample on a same vertical line as the first prediction sample.

In a method and an apparatus for encoding/decoding a video signal according to the present invention, each of the reference samples may be included in a different reference line.

The features briefly summarized above for the present invention are only illustrative aspects of the detailed description of the invention that follows, but do not limit the scope of the invention.

Advantageous Effects

According to the present invention, intra-prediction may be performed efficiently for an encoding/decoding target block.

According to the present invention, intra prediction can be performed based on a weighted prediction using a plurality of reference samples.

According to the present invention, encoding/decoding efficiency may be improved by refining a prediction sample generated through intra prediction.

The effects obtainable by the present invention are not limited to the above-mentioned effects, and other effects not mentioned can be clearly understood by those skilled in the art from the description below.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a device for encoding a video according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a device for decoding a video according to an embodiment of the present invention.

FIG. 3 is a diagram illustrating an example of hierarchically partitioning a coding block based on a tree structure according to an embodiment of the present invention.

FIG. 4 is a diagram illustrating a partition type in which binary tree-based partitioning is allowed according to an embodiment of the present invention.

FIGS. 5A and 5B are diagrams illustrating an example in which only a binary tree-based partition of a pre-determined type is allowed according to an embodiment of the present invention.

FIG. 6 is a diagram for explaining an example in which information related to the allowable number of binary tree partitioning is encoded/decoded, according to an embodiment to which the present invention is applied.

FIG. 7 is a diagram illustrating a partition mode applicable to a coding block according to an embodiment of the present invention.

FIG. 8 is a diagram illustrating types of pre-defined intra prediction modes for a device for encoding/decoding a video according to an embodiment of the present invention.

FIG. 9 is a diagram illustrating a kind of extended intra prediction modes according to an embodiment of the present invention.

FIG. 10 is a flowchart briefly illustrating an intra prediction method according to an embodiment of the present invention.

FIG. 11 is a diagram illustrating a method of correcting a prediction sample of a current block based on differential information of neighboring samples according to an embodiment of the present invention.

FIGS. 12 and 13 are diagrams illustrating a method of correcting a prediction sample based on a predetermined correction filter according to an embodiment of the present invention.

FIG. 14 shows a range of reference samples for intra prediction according to an embodiment to which the present invention is applied.

FIG. 15 is a diagram exemplifying a plurality of reference sample lines.

FIG. 16 is a flowchart illustrating a method for refining a prediction sample according to an embodiment of the present invention.

FIG. 17 is a flowchart illustrating a method for refining a prediction image in a unit of a sub-block according to an embodiment of the present invention.

FIGS. 18 to 22 are diagrams illustrating intra prediction patterns of a current block according to an embodiment to which the present invention is applied.

FIGS. 23 and 24 are diagrams illustrating examples of applying a different offset in a predetermined unit in a sub-block.

FIG. 25 is a flowchart illustrating processes of obtaining a residual sample according to an embodiment to which the present invention is applied.

MODE FOR INVENTION

A variety of modifications may be made to the present invention and there are various embodiments of the present invention, examples of which will now be provided with reference to drawings and described in detail. However, the present invention is not limited thereto, and the exemplary embodiments can be construed as including all modifications, equivalents, or substitutes in a technical concept and a technical scope of the present invention. The similar reference numerals refer to the similar element in described the drawings.

Terms used in the specification, ‘first’, ‘second’, etc. can be used to describe various components, but the components are not to be construed as being limited to the terms. The terms are only used to differentiate one component from other components. For example, the ‘first’ component may be named the ‘second’ component without departing from the scope of the present invention, and the ‘second’ component may also be similarly named the ‘first’ component. The term ‘and/or’ includes a combination of a plurality of items or any one of a plurality of terms.

It will be understood that when an element is simply referred to as being ‘connected to’ or ‘coupled to’ another element without being ‘directly connected to’ or ‘directly coupled to’ another element in the present description, it may be ‘directly connected to’ or ‘directly coupled to’ another element or be connected to or coupled to another element, having the other element intervening therebetween. In contrast, it should be understood that when an element is referred to as being “directly coupled” or “directly connected” to another element, there are no intervening elements present.

The terms used in the present specification are merely used to describe particular embodiments, and are not intended to limit the present invention. An expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. In the present specification, it is to be understood that terms such as “including”, “having”, etc. are intended to indicate the existence of the features, numbers, steps, actions, elements, parts, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, numbers, steps, actions, elements, parts, or combinations thereof may exist or may be added.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Hereinafter, the same constituent elements in the drawings are denoted by the same reference numerals, and a repeated description of the same elements will be omitted.

FIG. 1 is a block diagram illustrating a device for encoding a video according to an embodiment of the present invention.

Referring to FIG. 1, the device 100 for encoding a video may include: a picture partitioning module 110, prediction modules 120 and 125, a transform module 130, a quantization module 135, a rearrangement module 160, an entropy encoding module 165, an inverse quantization module 140, an inverse transform module 145, a filter module 150, and a memory 155.

The constitutional parts shown in FIG. 1 are independently shown so as to represent characteristic functions different from each other in the device for encoding a video. Thus, it does not mean that each constitutional part is constituted in a constitutional unit of separated hardware or software. In other words, each constitutional part includes each of enumerated constitutional parts for convenience. Thus, at least two constitutional parts of each constitutional part may be combined to form one constitutional part or one constitutional part may be divided into a plurality of constitutional parts to perform each function. The embodiment where each constitutional part is combined and the embodiment where one constitutional part is divided are also included in the scope of the present invention, if not departing from the essence of the present invention.

Also, some of constituents may not be indispensable constituents performing essential functions of the present invention but be selective constituents improving only performance thereof. The present invention may be implemented by including only the indispensable constitutional parts for implementing the essence of the present invention except the constituents used in improving performance. The structure including only the indispensable constituents except the selective constituents used in improving only performance is also included in the scope of the present invention.

The picture partitioning module 110 may partition an input picture into one or more processing units. Here, the processing unit may be a prediction unit (PU), a transform unit (TU), or a coding unit (CU). The picture partitioning module 110 may partition one picture into combinations of multiple coding units, prediction units, and transform units, and may encode a picture by selecting one combination of coding units, prediction units, and transform units with a predetermined criterion (e.g., cost function).

For example, one picture may be partitioned into multiple coding units. A recursive tree structure, such as a quad tree structure, may be used to partition a picture into coding units. A coding unit which is partitioned into other coding units with one picture or a largest coding unit as a root may be partitioned with child nodes corresponding to the number of partitioned coding units. A coding unit which is no longer partitioned by a predetermined limitation serves as a leaf node. That is, when it is assumed that only square partitioning is possible for one coding unit, one coding unit may be partitioned into four other coding units at most.

Hereinafter, in the embodiment of the present invention, the coding unit may mean a unit performing encoding, or a unit performing decoding.

A prediction unit may be one of partitions partitioned into a square or a rectangular shape having the same size in a single coding unit, or a prediction unit may be one of partitions partitioned so as to have a different shape/size in a single coding unit.

When a prediction unit subjected to intra prediction is generated based on a coding unit and the coding unit is not the smallest coding unit, intra prediction may be performed without partitioning the coding unit into multiple prediction units N×N.

The prediction modules 120 and 125 may include an inter prediction module 120 performing inter prediction and an intra prediction module 125 performing intra prediction. Whether to perform inter prediction or intra prediction for the prediction unit may be determined, and detailed information (e.g., an intra prediction mode, a motion vector, a reference picture, etc.) according to each prediction method may be determined. Here, the processing unit subjected to prediction may be different from the processing unit for which the prediction method and detailed content is determined. For example, the prediction method, the prediction mode, etc. may be determined by the prediction unit, and prediction may be performed by the transform unit. A residual value (residual block) between the generated prediction block and an original block may be input to the transform module 130. Also, prediction mode information, motion vector information, etc. used for prediction may be encoded with the residual value by the entropy encoding module 165 and may be transmitted to a device for decoding a video. When a particular encoding mode is used, it is possible to transmit to a device for decoding video by encoding the original block as it is without generating the prediction block through the prediction modules 120 and 125.

The inter prediction module 120 may predict the prediction unit based on information of at least one of a previous picture or a subsequent picture of the current picture, or may predict the prediction unit based on information of some encoded regions in the current picture, in some cases. The inter prediction module 120 may include a reference picture interpolation module, a motion prediction module, and a motion compensation module.

The reference picture interpolation module may receive reference picture information from the memory 155 and may generate pixel information of an integer pixel or less then the integer pixel from the reference picture. In the case of luma pixels, an 8-tap DCT-based interpolation filter having different filter coefficients may be used to generate pixel information of an integer pixel or less than an integer pixel in units of a ¼ pixel. In the case of chroma signals, a 4-tap DCT-based interpolation filter having different filter coefficient may be used to generate pixel information of an integer pixel or less than an integer pixel in units of a ⅛ pixel.

The motion prediction module may perform motion prediction based on the reference picture interpolated by the reference picture interpolation module. As methods for calculating a motion vector, various methods, such as a full search-based block matching algorithm (FBMA), a three step search (TSS), a new three-step search algorithm (NTS), etc., may be used. The motion vector may have a motion vector value in units of a ½ pixel or a ¼ pixel based on an interpolated pixel. The motion prediction module may predict a current prediction unit by changing the motion prediction method. As motion prediction methods, various methods, such as a skip method, a merge method, an AMVP (Advanced Motion Vector Prediction) method, an intra block copy method, etc., may be used.

The intra prediction module 125 may generate a prediction unit based on reference pixel information neighboring to a current block which is pixel information in the current picture. When the neighboring block of the current prediction unit is a block subjected to inter prediction and thus a reference pixel is a pixel subjected to inter prediction, the reference pixel included in the block subjected to inter prediction may be replaced with reference pixel information of a neighboring block subjected to intra prediction. That is, when a reference pixel is not available, at least one reference pixel of available reference pixels may be used instead of unavailable reference pixel information.

Prediction modes in intra prediction may include a directional prediction mode using reference pixel information depending on a prediction direction and a non-directional prediction mode not using directional information in performing prediction. A mode for predicting luma information may be different from a mode for predicting chroma information, and in order to predict the chroma information, intra prediction mode information used to predict luma information or predicted luma signal information may be utilized.

In performing intra prediction, when the size of the prediction unit is the same as the size of the transform unit, intra prediction may be performed on the prediction unit based on pixels positioned at the left, the top left, and the top of the prediction unit. However, in performing intra prediction, when the size of the prediction unit is different from the size of the transform unit, intra prediction may be performed using a reference pixel based on the transform unit. Also, intra prediction using N×N partitioning may be used for only the smallest coding unit.

In the intra prediction method, a prediction block may be generated after applying an AIS (Adaptive Intra Smoothing) filter to a reference pixel depending on the prediction modes. The type of the AIS filter applied to the reference pixel may vary. In order to perform the intra prediction method, an intra prediction mode of the current prediction unit may be predicted from the intra prediction mode of the prediction unit neighboring to the current prediction unit. In prediction of the prediction mode of the current prediction unit by using mode information predicted from the neighboring prediction unit, when the intra prediction mode of the current prediction unit is the same as the intra prediction mode of the neighboring prediction unit, information indicating that the prediction modes of the current prediction unit and the neighboring prediction unit are equal to each other may be transmitted using predetermined flag information. When the prediction mode of the current prediction unit is different from the prediction mode of the neighboring prediction unit, entropy encoding may be performed to encode prediction mode information of the current block.

Also, a residual block including information on a residual value which is a different between the prediction unit subjected to prediction and the original block of the prediction unit may be generated based on prediction units generated by the prediction modules 120 and 125. The generated residual block may be input to the transform module 130

The transform module 130 may transform the residual block including the information on the residual value between the original block and the prediction unit generated by the prediction modules 120 and 125 by using a transform method, such as discrete cosine transform (DCT), discrete sine transform (DST), and KLT. Whether to apply DCT, DST, or KLT in order to transform the residual block may be determined based on intra prediction mode information of the prediction unit used to generate the residual block.

The quantization module 135 may quantize values transformed to a frequency domain by the transform module 130. Quantization coefficients may vary depending on the block or importance of a picture. The values calculated by the quantization module 135 may be provided to the inverse quantization module 140 and the rearrangement module 160.

The rearrangement module 160 may rearrange coefficients of quantized residual values.

The rearrangement module 160 may change a coefficient in the form of a two-dimensional block into a coefficient in the form of a one-dimensional vector through a coefficient scanning method. For example, the rearrangement module 160 may scan from a DC coefficient to a coefficient in a high frequency domain using a zigzag scanning method so as to change the coefficients to be in the form of one-dimensional vectors. Depending on the size of the transform unit and the intra prediction mode, vertical direction scanning where coefficients in the form of two-dimensional blocks are scanned in the column direction or horizontal direction scanning where coefficients in the form of two-dimensional blocks are scanned in the row direction may be used instead of zigzag scanning. That is, which scanning method among zigzag scanning, vertical direction scanning, and horizontal direction scanning is used may be determined depending on the size of the transform unit and the intra prediction mode.

The entropy encoding module 165 may perform entropy encoding based on the values calculated by the rearrangement module 160. Entropy encoding may use various encoding methods, for example, exponential Golomb coding, context-adaptive variable length coding (CAVLC), and context-adaptive binary arithmetic coding (CABAC).

The entropy encoding module 165 may encode a variety of information, such as residual value coefficient information and block type information of the coding unit, prediction mode information, partition unit information, prediction unit information, transform unit information, motion vector information, reference frame information, block interpolation information, filtering information, etc. from the rearrangement module 160 and the prediction modules 120 and 125.

The entropy encoding module 165 may entropy encode the coefficients of the coding unit input from the rearrangement module 160.

The inverse quantization module 140 may inversely quantize the values quantized by the quantization module 135 and the inverse transform module 145 may inversely transform the values transformed by the transform module 130. The residual value generated by the inverse quantization module 140 and the inverse transform module 145 may be combined with the prediction unit predicted by a motion estimation module, a motion compensation module, and the intra prediction module of the prediction modules 120 and 125 such that a reconstructed block can be generated.

The filter module 150 may include at least one of a deblocking filter, an offset correction unit, and an adaptive loop filter (ALF).

The deblocking filter may remove block distortion that occurs due to boundaries between the blocks in the reconstructed picture. In order to determine whether to perform deblocking, the pixels included in several rows or columns in the block may be a basis of determining whether to apply the deblocking filter to the current block. When the deblocking filter is applied to the block, a strong filter or a weak filter may be applied depending on required deblocking filtering strength. Also, in applying the deblocking filter, horizontal direction filtering and vertical direction filtering may be processed in parallel.

The offset correction module may correct offset with the original picture in units of a pixel in the picture subjected to deblocking. In order to perform the offset correction on a particular picture, it is possible to use a method of applying offset in consideration of edge information of each pixel or a method of partitioning pixels of a picture into the predetermined number of regions, determining a region to be subjected to perform offset, and applying the offset to the determined region.

Adaptive loop filtering (ALF) may be performed based on the value obtained by comparing the filtered reconstructed picture and the original picture. The pixels included in the picture may be divided into predetermined groups, a filter to be applied to each of the groups may be determined, and filtering may be individually performed for each group. Information on whether to apply ALF and a luma signal may be transmitted by coding units (CU). The shape and filter coefficient of a filter for ALF may vary depending on each block. Also, the filter for ALF in the same shape (fixed shape) may be applied regardless of characteristics of the application target block.

The memory 155 may store the reconstructed block or picture calculated through the filter module 150. The stored reconstructed block or picture may be provided to the prediction modules 120 and 125 in performing inter prediction.

FIG. 2 is a block diagram illustrating a device for decoding a video according to an embodiment of the present invention.

Referring to FIG. 2, the device 200 for decoding a video may include: an entropy decoding module 210, a rearrangement module 215, an inverse quantization module 220, an inverse transform module 225, prediction modules 230 and 235, a filter module 240, and a memory 245.

When a video bitstream is input from the device for encoding a video, the input bitstream may be decoded according to an inverse process of the device for encoding a video.

The entropy decoding module 210 may perform entropy decoding according to an inverse process of entropy encoding by the entropy encoding module of the device for encoding a video. For example, corresponding to the methods performed by the device for encoding a video, various methods, such as exponential Golomb coding, context-adaptive variable length coding (CAVLC), and context-adaptive binary arithmetic coding (CABAC) may be applied.

The entropy decoding module 210 may decode information on intra prediction and inter prediction performed by the device for encoding a video.

The rearrangement module 215 may perform rearrangement on the bitstream entropy decoded by the entropy decoding module 210 based on the rearrangement method used in the device for encoding a video. The rearrangement module may reconstruct and rearrange the coefficients in the form of one-dimensional vectors to the coefficient in the form of two-dimensional blocks. The rearrangement module 215 may receive information related to coefficient scanning performed in the device for encoding a video and may perform rearrangement via a method of inversely scanning the coefficients based on the scanning order performed in the device for encoding a video.

The inverse quantization module 220 may perform inverse quantization based on a quantization parameter received from the device for encoding a video and the rearranged coefficients of the block.

The inverse transform module 225 may perform the inverse transform, i.e., inverse DCT, inverse DST, and inverse KLT, which is the inverse process of transform, i.e., DCT, DST, and KLT, performed by the transform module on the quantization result by the device for encoding a video. Inverse transform may be performed based on a transfer unit determined by the device for encoding a video. The inverse transform module 225 of the device for decoding a video may selectively perform transform schemes (e.g., DCT, DST, and KLT) depending on multiple pieces of information, such as the prediction method, the size of the current block, the prediction direction, etc.

The prediction modules 230 and 235 may generate a prediction block based on information on prediction block generation received from the entropy decoding module 210 and previously decoded block or picture information received from the memory 245.

As described above, like the operation of the device for encoding a video, in performing intra prediction, when the size of the prediction unit is the same as the size of the transform unit, intra prediction may be performed on the prediction unit based on the pixels positioned at the left, the top left, and the top of the prediction unit. In performing intra prediction, when the size of the prediction unit is different from the size of the transform unit, intra prediction may be performed using a reference pixel based on the transform unit. Also, intra prediction using N×N partitioning may be used for only the smallest coding unit.

The prediction modules 230 and 235 may include a prediction unit determination module, an inter prediction module, and an intra prediction module. The prediction unit determination module may receive a variety of information, such as prediction unit information, prediction mode information of an intra prediction method, information on motion prediction of an inter prediction method, etc. from the entropy decoding module 210, may divide a current coding unit into prediction units, and may determine whether inter prediction or intra prediction is performed on the prediction unit. By using information required in inter prediction of the current prediction unit received from the device for encoding a video, the inter prediction module 230 may perform inter prediction on the current prediction unit based on information of at least one of a previous picture or a subsequent picture of the current picture including the current prediction unit. Alternatively, inter prediction may be performed based on information of some pre-reconstructed regions in the current picture including the current prediction unit.

In order to perform inter prediction, it may be determined for the coding unit which of a skip mode, a merge mode, an AMVP mode, and an inter block copy mode is used as the motion prediction method of the prediction unit included in the coding unit.

The intra prediction module 235 may generate a prediction block based on pixel information in the current picture. When the prediction unit is a prediction unit subjected to intra prediction, intra prediction may be performed based on intra prediction mode information of the prediction unit received from the device for encoding a video. The intra prediction module 235 may include an adaptive intra smoothing (AIS) filter, a reference pixel interpolation module, and a DC filter. The AIS filter performs filtering on the reference pixel of the current block, and whether to apply the filter may be determined depending on the prediction mode of the current prediction unit. AIS filtering may be performed on the reference pixel of the current block by using the prediction mode of the prediction unit and AIS filter information received from the device for encoding a video. When the prediction mode of the current block is a mode where AIS filtering is not performed, the AIS filter may not be applied.

When the prediction mode of the prediction unit is a prediction mode in which intra prediction is performed based on the pixel value obtained by interpolating the reference pixel, the reference pixel interpolation module may interpolate the reference pixel to generate the reference pixel of an integer pixel or less than an integer pixel. When the prediction mode of the current prediction unit is a prediction mode in which a prediction block is generated without interpolation the reference pixel, the reference pixel may not be interpolated. The DC filter may generate a prediction block through filtering when the prediction mode of the current block is a DC mode.

The reconstructed block or picture may be provided to the filter module 240. The filter module 240 may include the deblocking filter, the offset correction module, and the ALF.

Information on whether or not the deblocking filter is applied to the corresponding block or picture and information on which of a strong filter and a weak filter is applied when the deblocking filter is applied may be received from the device for encoding a video. The deblocking filter of the device for decoding a video may receive information on the deblocking filter from the device for encoding a video, and may perform deblocking filtering on the corresponding block.

The offset correction module may perform offset correction on the reconstructed picture based on the type of offset correction and offset value information applied to a picture in performing encoding.

The ALF may be applied to the coding unit based on information on whether to apply the ALF, ALF coefficient information, etc. received from the device for encoding a video. The ALF information may be provided as being included in a particular parameter set.

The memory 245 may store the reconstructed picture or block for use as a reference picture or block, and may provide the reconstructed picture to an output module.

As described above, in the embodiment of the present invention, for convenience of explanation, the coding unit is used as a term representing a unit for encoding, but the coding unit may serve as a unit performing decoding as well as encoding.

In addition, a current block may represent a target block to be encoded/decoded. And, the current block may represent a coding tree block (or a coding tree unit), a coding block (or a coding unit), a transform block (or a transform unit), a prediction block (or a prediction unit), or the like depending on an encoding/decoding step.

A picture may be encoded/decoded by divided into base blocks having a square shape or a non-square shape. At this time, the base block may be referred to as a coding tree unit. The coding tree unit may be defined as a coding unit of the largest size allowed within a sequence or a slice. Information regarding whether the coding tree unit has a square shape or has a non-square shape or information regarding a size of the coding tree unit may be signaled through a sequence parameter set, a picture parameter set, or a slice header. The coding tree unit may be divided into smaller size partitions. At this time, if it is assumed that a depth of a partition generated by dividing the coding tree unit is 1, a depth of a partition generated by dividing the partition having depth 1 may be defined as 2. That is, a partition generated by dividing a partition having a depth k in the coding tree unit may be defined as having a depth k+1.

A partition of arbitrary size generated by dividing a coding tree unit may be defined as a coding unit. The coding unit may be recursively divided or divided into base units for performing prediction, quantization, transform, or in-loop filtering, and the like. For example, a partition of arbitrary size generated by dividing the coding unit may be defined as a coding unit, or may be defined as a transform unit or a prediction unit, which is a base unit for performing prediction, quantization, transform or in-loop filtering and the like.

Partitioning of a coding tree unit or a coding unit may be performed based on at least one of a vertical line and a horizontal line. In addition, the number of vertical lines or horizontal lines partitioning the coding tree unit or the coding unit may be at least one or more. For example, the coding tree unit or the coding unit may be divided into two partitions using one vertical line or one horizontal line, or the coding tree unit or the coding unit may be divided into three partitions using two vertical lines or two horizontal lines. Alternatively, the coding tree unit or the coding unit may be partitioned into four partitions having a length and a width of ½ by using one vertical line and one horizontal line.

When a coding tree unit or a coding unit is divided into a plurality of partitions using at least one vertical line or at least one horizontal line, the partitions may have a uniform size or a different size. Alternatively, any one partition may have a different size from the remaining partitions.

In the embodiments described below, it is assumed that a coding tree unit or a coding unit is divided into a quad tree structure or a binary tree structure. However, it is also possible to divide a coding tree unit or a coding unit using a larger number of vertical lines or a larger number of horizontal lines.

FIG. 3 is a diagram illustrating an example of hierarchically partitioning a coding block based on a tree structure according to an embodiment of the present invention.

An input video signal is decoded in predetermined block units. Such a default unit for decoding the input video signal is a coding block. The coding block may be a unit performing intra/inter prediction, transform, and quantization. In addition, a prediction mode (e.g., intra prediction mode or inter prediction mode) is determined in units of a coding block, and the prediction blocks included in the coding block may share the determined prediction mode. The coding block may be a square or non-square block having an arbitrary size in a range of 8×8 to 64×64, or may be a square or non-square block having a size of 128×128, 256×256, or more.

Specifically, the coding block may be hierarchically partitioned based on at least one of a quad tree and a binary tree. Here, quad tree-based partitioning may mean that a 2N×2N coding block is partitioned into four N×N coding blocks, and binary tree-based partitioning may mean that one coding block is partitioned into two coding blocks. Even if the binary tree-based partitioning is performed, a square-shaped coding block may exist in the lower depth.

Binary tree-based partitioning may be symmetrically or asymmetrically performed. The coding block partitioned based on the binary tree may be a square block or a non-square block, such as a rectangular shape. For example, a partition type in which the binary tree-based partitioning is allowed may comprise at least one of a symmetric type of 2N×N (horizontal directional non-square coding unit) or N×2N (vertical direction non-square coding unit), asymmetric type of nL×2N, nR×2N, 2N×nU, or 2N×nD.

Binary tree-based partitioning may be limitedly allowed to one of a symmetric or an asymmetric type partition. In this case, constructing the coding tree unit with square blocks may correspond to quad tree CU partitioning, and constructing the coding tree unit with symmetric non-square blocks may correspond to binary tree partitioning. Constructing the coding tree unit with square blocks and symmetric non-square blocks may correspond to quad and binary tree CU partitioning.

Binary tree-based partitioning may be performed on a coding block where quad tree-based partitioning is no longer performed. Quad tree-based partitioning may no longer be performed on the coding block partitioned based on the binary tree.

Furthermore, partitioning of a lower depth may be determined depending on a partition type of an upper depth. For example, if binary tree-based partitioning is allowed in two or more depths, only the same type as the binary tree partitioning of the upper depth may be allowed in the lower depth. For example, if the binary tree-based partitioning in the upper depth is performed with 2N×N type, the binary tree-based partitioning in the lower depth is also performed with 2N×N type. Alternatively, if the binary tree-based partitioning in the upper depth is performed with N×2N type, the binary tree-based partitioning in the lower depth is also performed with N×2N type.

On the contrary, it is also possible to allow, in a lower depth, only a type different from a binary tree partitioning type of an upper depth.

It may be possible to limit only a specific type of binary tree based partitioning to be used for sequence, slice, coding tree unit, or coding unit. As an example, only 2N×N type or N×2N type of binary tree-based partitioning may be allowed for the coding tree unit. An available partition type may be predefined in an encoder or a decoder. Or information on available partition type or on unavailable partition type on may be encoded and then signaled through a bitstream.

FIGS. 5A and 5B are diagrams illustrating an example in which only a specific type of binary tree-based partitioning is allowed. FIG. 5A shows an example in which only N×2N type of binary tree-based partitioning is allowed, and FIG. 5B shows an example in which only 2N×N type of binary tree-based partitioning is allowed. In order to implement adaptive partitioning based on the quad tree or binary tree, information indicating quad tree-based partitioning, information on the size/depth of the coding block that quad tree-based partitioning is allowed, information indicating binary tree-based partitioning, information on the size/depth of the coding block that binary tree-based partitioning is allowed, information on the size/depth of the coding block that binary tree-based partitioning is not allowed, information on whether binary tree-based partitioning is performed in a vertical direction or a horizontal direction, etc. may be used.

In addition, information on the number of times a binary tree partitioning is allowed, a depth at which the binary tree partitioning is allowed, or the number of the depths at which the binary tree partitioning is allowed may be obtained for a coding tree unit or a specific coding unit. The information may be encoded in units of a coding tree unit or a coding unit, and may be transmitted to a decoder through a bitstream.

For example, a syntax ‘max_binary_depth_idx_minus1’ indicating a maximum depth at which binary tree partitioning is allowed may be encoded/decoded through a bitstream. In this case, max_binary_depth_idx_minus1+1 may indicate the maximum depth at which the binary tree partitioning is allowed.

Referring to the example shown in FIG. 6, in FIG. 6, the binary tree partitioning has been performed for a coding unit having a depth of 2 and a coding unit having a depth of 3. Accordingly, at least one of information indicating the number of times the binary tree partitioning in the coding tree unit has been performed (i.e., 2 times), information indicating the maximum depth which the binary tree partitioning has been allowed in the coding tree unit (i.e., depth 3), or the number of depths in which the binary tree partitioning has been performed in the coding tree unit (i.e., 2 (depth 2 and depth 3)) may be encoded/decoded through a bitstream.

As another example, at least one of information on the number of times the binary tree partitioning is permitted, the depth at which the binary tree partitioning is allowed, or the number of the depths at which the binary tree partitioning is allowed may be obtained for each sequence or each slice. For example, the information may be encoded in units of a sequence, a picture, or a slice unit and transmitted through a bitstream. Accordingly, at least one of the number of the binary tree partitioning in a first slice, the maximum depth in which the binary tree partitioning is allowed in the first slice, or the number of depths in which the binary tree partitioning is performed in the first slice may be difference from a second slice. For example, in the first slice, binary tree partitioning may be permitted for only one depth, while in the second slice, binary tree partitioning may be permitted for two depths.

As another example, the number of times the binary tree partitioning is permitted, the depth at which the binary tree partitioning is allowed, or the number of depths at which the binary tree partitioning is allowed may be set differently according to a time level identifier (TemporalID) of a slice or a picture. Here, the temporal level identifier (TemporalID) is used to identify each of a plurality of layers of video having a scalability of at least one of view, spatial, temporal or quality.

As shown in FIG. 3, the first coding block 300 with the partition depth (split depth) of k may be partitioned into multiple second coding blocks based on the quad tree. For example, the second coding blocks 310 to 340 may be square blocks having the half width and the half height of the first coding block, and the partition depth of the second coding block may be increased to k+1.

The second coding block 310 with the partition depth of k+1 may be partitioned into multiple third coding blocks with the partition depth of k+2. Partitioning of the second coding block 310 may be performed by selectively using one of the quad tree and the binary tree depending on a partitioning method. Here, the partitioning method may be determined based on at least one of the information indicating quad tree-based partitioning and the information indicating binary tree-based partitioning.

When the second coding block 310 is partitioned based on the quad tree, the second coding block 310 may be partitioned into four third coding blocks 310 a having the half width and the half height of the second coding block, and the partition depth of the third coding block 310 a may be increased to k+2. In contrast, when the second coding block 310 is partitioned based on the binary tree, the second coding block 310 may be partitioned into two third coding blocks. Here, each of two third coding blocks may be a non-square block having one of the half width and the half height of the second coding block, and the partition depth may be increased to k+2. The second coding block may be determined as a non-square block of a horizontal direction or a vertical direction depending on a partitioning direction, and the partitioning direction may be determined based on the information on whether binary tree-based partitioning is performed in a vertical direction or a horizontal direction.

In the meantime, the second coding block 310 may be determined as a leaf coding block that is no longer partitioned based on the quad tree or the binary tree. In this case, the leaf coding block may be used as a prediction block or a transform block.

Like partitioning of the second coding block 310, the third coding block 310 a may be determined as a leaf coding block, or may be further partitioned based on the quad tree or the binary tree.

In the meantime, the third coding block 310 b partitioned based on the binary tree may be further partitioned into coding blocks 310 b-2 of a vertical direction or coding blocks 310 b-3 of a horizontal direction based on the binary tree, and the partition depth of the relevant coding blocks may be increased to k+3. Alternatively, the third coding block 310 b may be determined as a leaf coding block 310 b-1 that is no longer partitioned based on the binary tree. In this case, the coding block 310 b-1 may be used as a prediction block or a transform block. However, the above partitioning process may be limitedly performed based on at least one of the information on the size/depth of the coding block that quad tree-based partitioning is allowed, the information on the size/depth of the coding block that binary tree-based partitioning is allowed, and the information on the size/depth of the coding block that binary tree-based partitioning is not allowed.

A number of a candidate that represent a size of a coding block may be limited to a predetermined number, or a size of a coding block in a predetermined unit may have a fixed value. As an example, the size of the coding block in a sequence or in a picture may be limited to have 256×256, 128×128, or 32×32. Information indicating the size of the coding block in the sequence or in the picture may be signaled through a sequence header or a picture header.

As a result of partitioning based on a quad tree and a binary tree, a coding unit may be represented as square or rectangular shape of an arbitrary size.

A coding block is encoded using at least one of a skip mode, intra prediction, inter prediction, or a skip method. Once a coding block is determined, a prediction block may be determined through predictive partitioning of the coding block. The predictive partitioning of the coding block may be performed by a partition mode (Part_mode) indicating a partition type of the coding block. A size or a shape of the prediction block may be determined according to the partition mode of the coding block. For example, a size of a prediction block determined according to the partition mode may be equal to or smaller than a size of a coding block.

FIG. 7 is a diagram illustrating a partition mode that may be applied to a coding block when the coding block is encoded by inter prediction.

When a coding block is encoded by inter prediction, one of 8 partitioning modes may be applied to the coding block, as in the example shown in FIG. 7.

When a coding block is encoded by intra prediction, a partition mode PART_2N×2N or a partition mode PART_N×N may be applied to the coding block.

PART_N×N may be applied when a coding block has a minimum size. Here, the minimum size of the coding block may be pre-defined in an encoder and a decoder. Or, information regarding the minimum size of the coding block may be signaled via a bitstream. For example, the minimum size of the coding block may be signaled through a slice header, so that the minimum size of the coding block may be defined per slice.

In general, a prediction block may have a size from 64×64 to 4×4. However, when a coding block is encoded by inter prediction, it may be restricted that the prediction block does not have a 4×4 size in order to reduce memory bandwidth when performing motion compensation.

FIG. 8 is a diagram illustrating types of pre-defined intra prediction modes for a device for encoding/decoding a video according to an embodiment of the present invention.

The device for encoding/decoding a video may perform intra prediction using one of pre-defined intra prediction modes. The pre-defined intra prediction modes for intra prediction may include non-directional prediction modes (e.g., a planar mode, a DC mode) and 33 directional prediction modes.

Alternatively, in order to enhance accuracy of intra prediction, a larger number of directional prediction modes than the 33 directional prediction modes may be used. That is, M extended directional prediction modes may be defined by subdividing angles of the directional prediction modes (M>33), and a directional prediction mode having a predetermined angle may be derived using at least one of the 33 pre-defined directional prediction modes.

A larger number of intra prediction modes than 35 intra prediction modes shown in FIG. 8 may be used. For example, a larger number of intra prediction modes than the 35 intra prediction modes can be used by subdividing angles of directional prediction modes or by deriving a directional prediction mode having a predetermined angle using at least one of a pre-defined number of directional prediction modes. At this time, the use of a larger number of intra prediction modes than the 35 intra prediction modes may be referred to as an extended intra prediction mode.

FIG. 9 shows an example of extended intra prediction modes, and the extended intra prediction modes may include two non-directional prediction modes and 65 extended directional prediction modes. The same numbers of the extended intra prediction modes may be used for a luma component and a chroma component, or a different number of intra prediction modes may be used for each component. For example, 67 extended intra prediction modes may be used for the luma component, and 35 intra prediction modes may be used for the chroma component.

Alternatively, depending on the chroma format, a different number of intra prediction modes may be used in performing intra prediction. For example, in the case of the 4:2:0 format, 67 intra prediction modes may be used for the luma component to perform intra prediction and 35 intra prediction modes may be used for the chroma component. In the case of the 4:4:4 format, 67 intra prediction modes may be used for both the luma component and the chroma component to perform intra prediction.

Alternatively, depending on the size and/or shape of the block, a different number of intra prediction modes may be used to perform intra prediction. That is, depending on the size and/or shape of the PU or CU, 35 intra prediction modes or 67 intra prediction modes may be used to perform intra prediction. For example, when the CU or PU has the size less than 64×64 or is asymmetrically partitioned, 35 intra prediction modes may be used to perform intra prediction. When the size of the CU or PU is equal to or greater than 64×64, 67 intra prediction modes may be used to perform intra prediction. 65 directional intra prediction modes may be allowed for Intra 2N×2N, and only 35 directional intra prediction modes may be allowed for Intra N×N.

A size of a block to which the extended intra prediction mode is applied may be set differently for each sequence, picture or slice. For example, it is set that the extended intra prediction mode is applied to a block (e.g., CU or PU) which has a size greater than 64×64 in the first slice. On the other hands, it is set that the extended intra prediction mode is applied to a block which has a size greater than 32×32 in the second slice. Information representing a size of a block to which the extended intra prediction mode is applied may be signaled through in units of a sequence, a picture, or a slice. For example, the information indicating the size of the block to which the extended intra prediction mode is applied may be defined as ‘log2_extended_intra_mode_size_minus4’ obtained by taking a logarithm of the block size and then subtracting the integer 4. For example, if a value of log2_extended_intra_mode_size_minus4 is 0, it may indicate that the extended intra prediction mode may be applied to a block having a size equal to or greater than 16×16. And if a value of log2_extended_intra_mode_size_minus4 is 1, it may indicate that the extended intra prediction mode may be applied to a block having a size equal to or greater than 32×32.

As described above, the number of intra prediction modes may be determined in consideration of at least one of a color component, a chroma format, and a size or a shape of a block. In addition, the number of intra prediction mode candidates (e.g., the number of MPMs) used for determining an intra prediction mode of a current block to be encoded/decoded may also be determined according to at least one of a color component, a color format, and the size or a shape of a block. A method of determining an intra prediction mode of a current block to be encoded/decoded and a method of performing intra prediction using the determined intra prediction mode will be described with the drawings.

FIG. 10 is a flowchart briefly illustrating an intra prediction method according to an embodiment of the present invention.

Encoding/decoding efficiency may be improved by applying an intra prediction to a block included in an image having strong directionality, a block included in an image not shown in a previous frame, or the like.

Referring to FIG. 10, an intra prediction mode of the current block may be determined at step S1000.

Specifically, the intra prediction mode of the current block may be derived based on a candidate list and an index. Here, the candidate list contains multiple candidates, and the multiple candidates may be determined based on an intra prediction mode of the neighboring block adjacent to the current block. The neighboring block may include at least one of blocks positioned at the top, the bottom, the left, the right, and the corner of the current block. The index may specify one of the multiple candidates of the candidate list. The candidate specified by the index may be set to the intra prediction mode of the current block.

An intra prediction mode used for intra prediction in the neighboring block may be set as a candidate. Also, an intra prediction mode having directionality similar to that of the intra prediction mode of the neighboring block may be set as a candidate. Here, the intra prediction mode having similar directionality may be determined by adding or subtracting a predetermined constant value to or from the intra prediction mode of the neighboring block. The predetermined constant value may be an integer, such as one, two, or more.

The candidate list may further include a default mode. The default mode may include at least one of a planar mode, a DC mode, a vertical mode, and a horizontal mode. The default mode may be adaptively added considering the maximum number of candidates that can be included in the candidate list of the current block.

The maximum number of candidates that can be included in the candidate list may be three, four, five, six, or more. The maximum number of candidates that can be included in the candidate list may be a fixed value preset in the device for encoding/decoding a video, or may be variably determined based on a characteristic of the current block. The characteristic may mean the location/size/shape of the block, the number/type of intra prediction modes that the block can use, a color type, a color format, etc. Alternatively, information indicating the maximum number of candidates that can be included in the candidate list may be signaled separately, and the maximum number of candidates that can be included in the candidate list may be variably determined using the information. The information indicating the maximum number of candidates may be signaled in at least one of a sequence level, a picture level, a slice level, and a block level.

When the extended intra prediction modes and the 35 pre-defined intra prediction modes are selectively used, the intra prediction modes of the neighboring blocks may be transformed into indexes corresponding to the extended intra prediction modes, or into indexes corresponding to the 35 intra prediction modes, whereby candidates can be derived. For transform to an index, a pre-defined table may be used, or a scaling operation based on a predetermined value may be used. Here, the pre-defined table may define a mapping relation between different intra prediction mode groups (e.g., extended intra prediction modes and 35 intra prediction modes).

For example, when the left neighboring block uses the 35 intra prediction modes and the intra prediction mode of the left neighboring block is 10 (a horizontal mode), it may be transformed into an index of 16 corresponding to a horizontal mode in the extended intra prediction modes.

Alternatively, when the top neighboring block uses the extended intra prediction modes and the intra prediction mode the top neighboring block has an index of 50 (a vertical mode), it may be transformed into an index of 26 corresponding to a vertical mode in the 35 intra prediction modes.

Based on the above-described method of determining the intra prediction mode, the intra prediction mode may be derived independently for each of the luma component and the chroma component, or the intra prediction mode of the chroma component may be derived depending on the intra prediction mode of the luma component.

Specifically, the intra prediction mode of the chroma component may be determined based on the intra prediction mode of the luma component as shown in the following Table 1.

TABLE 1 IntraPredModeY[xCb][yCb] Intra_chroma_pred_mode[xCb][yCb] 0 26 10 1 X(0 <= X <= 34) 0 34 0 0 0 0 1 26 34 26 26 26 2 10 10 34 10 10 3 1 1 1 34 1 4 0 26 10 1 X

In Table 1, intra chroma pred mode means information signaled to specify the intra prediction mode of the chroma component, and IntraPredModeY indicates the intra prediction mode of the luma component.

Referring to FIG. 10, a reference sample for intra prediction of the current block may be derived at step S1010.

Specifically, a reference sample for intra prediction may be derived based on a neighboring sample of the current block. The neighboring sample may be a reconstructed sample of the neighboring block, and the reconstructed sample may be a reconstructed sample before an in-loop filter is applied or a reconstructed sample after the in-loop filter is applied.

A neighboring sample reconstructed before the current block may be used as the reference sample, and a neighboring sample filtered based on a predetermined intra filter may be used as the reference sample. Filtering of neighboring samples using an intra filter may also be referred to as reference sample smoothing. The intra filter may include at least one of the first intra filter applied to multiple neighboring samples positioned on the same horizontal line and the second intra filter applied to multiple neighboring samples positioned on the same vertical line. Depending on the positions of the neighboring samples, one of the first intra filter and the second intra filter may be selectively applied, or both intra filters may be applied. At this time, at least one filter coefficient of the first intra filter or the second intra filter may be (1, 2, 1), but is not limited thereto.

Filtering may be adaptively performed based on at least one of the intra prediction mode of the current block and the size of the transform block for the current block. For example, when the intra prediction mode of the current block is the DC mode, the vertical mode, or the horizontal mode, filtering may not be performed. When the size of the transform block is N×M, filtering may not be performed. Here, N and M may be the same values or different values, or may be values of 4, 8, 16, or more. For example, if the size of the transform block is 4×4, filtering may not be performed. Alternatively, filtering may be selectively performed based on the result of a comparison of a pre-defined threshold and the difference between the intra prediction mode of the current block and the vertical mode (or the horizontal mode). For example, when the difference between the intra prediction mode of the current block and the vertical mode is greater than a threshold, filtering may be performed. The threshold may be defined for each size of the transform block as shown in Table 2.

TABLE 2 8 × 8 16 × 16 32 × 32 transform transform transform Threshold 7 1 0

The intra filter may be determined as one of multiple intra filter candidates pre-defined in the device for encoding/decoding a video. To this end, an index specifying an intra filter of the current block among the multiple intra filter candidates may be signaled. Alternatively, the intra filter may be determined based on at least one of the size/shape of the current block, the size/shape of the transform block, information on the filter strength, and variations of the neighboring samples.

Referring to FIG. 10, intra prediction may be performed using the intra prediction mode of the current block and the reference sample at step S1020.

That is, the prediction sample of the current block may be obtained using the intra prediction mode determined at step S1000 and the reference sample derived at step S1010. However, in the case of intra prediction, a boundary sample of the neighboring block may be used, and thus quality of the prediction picture may be decreased. Therefore, a correction process may be performed on the prediction sample generated through the above-described prediction process, and will be described in detail with reference to FIGS. 11 to 13. However, the correction process is not limited to being applied only to the intra prediction sample, and may be applied to an inter prediction sample or the reconstructed sample.

FIG. 11 is a diagram illustrating a method of correcting a prediction sample of a current block based on differential information of neighboring samples according to an embodiment of the present invention.

The prediction sample of the current block may be corrected based on the differential information of multiple neighboring samples for the current block. The correction may be performed on all prediction samples in the current block, or may be performed on prediction samples in predetermined partial regions. The partial regions may be one row/column or multiple rows/columns, and these may be preset regions for correction in the device for encoding/decoding a video. For example, correction may be performed on a one row/column located at a boundary of the current block or may be performed on plurality of rows/columns from a boundary of the current block. Alternatively, the partial regions may be variably determined based on at least one of the size/shape of the current block and the intra prediction mode.

The neighboring samples may belong to the neighboring blocks positioned at the top, the left, and the top left corner of the current block. The number of neighboring samples used for correction may be two, three, four, or more. The positions of the neighboring samples may be variably determined depending on the position of the prediction sample which is the correction target in the current block. Alternatively, some of the neighboring samples may have fixed positions regardless of the position of the prediction sample which is the correction target, and the remaining neighboring samples may have variable positions depending on the position of the prediction sample which is the correction target.

The differential information of the neighboring samples may mean a differential sample between the neighboring samples, or may mean a value obtained by scaling the differential sample by a predetermined constant value (e.g., one, two, three, etc.). Here, the predetermined constant value may be determined considering the position of the prediction sample which is the correction target, the position of the column or row including the prediction sample which is the correction target, the position of the prediction sample within the column or row, etc.

For example, when the intra prediction mode of the current block is the vertical mode, differential samples between the top left neighboring sample p(−1, −1) and neighboring samples p (−1, y) adjacent to the left boundary of the current block may be used to obtain the final prediction sample as shown in Equation 1.

P′(0,y)=P(0,y)+((p(−1,y)−p(−1,−1))>>1 for y=0 . . . N−1   [Equation 1]

For example, when the intra prediction mode of the current block is the horizontal mode, differential samples between the top left neighboring sample p(−1, −1) and neighboring samples p(x, −1) adjacent to the top boundary of the current block may be used to obtain the final prediction sample as shown in Equation 2.

P′(x,0)=p(x ,0)+((p(x,−1)−p(−1,−1))>>1 for x=0 . . . N−1   [Equation 2]

For example, when the intra prediction mode of the current block is the vertical mode, differential samples between the top left neighboring sample p(−1, −1) and neighboring samples p(−1, y) adjacent to the left boundary of the current block may be used to obtain the final prediction sample. Here, the differential sample may be added to the prediction sample, or the differential sample may be scaled by a predetermined constant value, and then added to the prediction sample. The predetermined constant value used in scaling may be determined differently depending on the column and/or row. For example, the prediction sample may be corrected as shown in Equation 3 and Equation 4.

P′(0,y)=P(0,y)+((p(−1,y)−p(−1,−1))>>1 for y=0 . . . N−1   [Equation 3]

P′(1,y)=P(1,y)+((p(−1,y)−p(−1,−1))>>2 for y=0 . . . N−1   [Equation 4]

For example, when the intra prediction mode of the current block is the horizontal mode, differential samples between the top left neighboring sample p(−1, −1) and neighboring samples p(x, −1) adjacent to the top boundary of the current block may be used to obtain the final prediction sample, as described in the case of the vertical mode. For example, the prediction sample may be corrected as shown in Equation 5 and Equation 6.

P′(x,0)=p(x ,0)+((p(x,−1)−p(−1,−1))>>1 for x=0 . . . N−1   [Equation 5]

P′(x,1)=p(x ,1)+((p(x,−1)−p(−1,−1))>>2 for x=0 . . . N−1   [Equation 6]

FIGS. 12 and 13 are diagrams illustrating a method of correcting a prediction sample based on a predetermined correction filter according to an embodiment of the present invention.

The prediction sample may be corrected based on the neighboring sample of the prediction sample which is the correction target and a predetermined correction filter. Here, the neighboring sample may be specified by an angular line of the directional prediction mode of the current block, or may be at least one sample positioned on the same angular line as the prediction sample which is the correction target. Also, the neighboring sample may be a prediction sample in the current block, or may be a reconstructed sample in a neighboring block reconstructed before the current block.

At least one of the number of taps, strength, and a filter coefficient of the correction filter may be determined based on at least one of the position of the prediction sample which is the correction target, whether or not the prediction sample which is the correction target is positioned on the boundary of the current block, the intra prediction mode of the current block, angle of the directional prediction mode, the prediction mode (inter or intra mode) of the neighboring block, and the size/shape of the current block.

Referring to FIG. 12, when the directional prediction mode has an index of 2 or 34, at least one prediction/reconstructed sample positioned at the bottom left of the prediction sample which is the correction target and the predetermined correction filter may be used to obtain the final prediction sample. Here, the prediction/reconstructed sample at the bottom left may belong to a previous line of a line including the prediction sample which is the correction target. The prediction/reconstructed sample at the bottom left may belong to the same block as the current sample, or to neighboring block adjacent to the current block.

Filtering for the prediction sample may be performed only on the line positioned at the block boundary, or may be performed on multiple lines. The correction filter where at least one of the number of filter taps and a filter coefficient is different for each of lines may be used. For example, a (1/2, 1/2) filter may be used for the left first line closest to the block boundary, a (12/16, 4/16) filter may be used for the second line, a (14/16, 2/16) filter may be used for the third line, and a (15/16, 1/16) filter may be used for the fourth line.

Alternatively, when the directional prediction mode has an index of 3 to 6 or 30 to 33, filtering may be performed on the block boundary as shown in FIG. 13, and a 3-tap correction filter may be used to correct the prediction sample. Filtering may be performed using the bottom left sample of the prediction sample which is the correction target, the bottom sample of the bottom left sample, and a 3-tap correction filter that takes as input the prediction sample which is the correction target. The position of neighboring sample used by the correction filter may be determined differently based on the directional prediction mode. The filter coefficient of the correction filter may be determined differently depending on the directional prediction mode.

Different correction filters may be applied depending on whether the neighboring block is encoded in the inter mode or the intra mode. When the neighboring block is encoded in the intra mode, a filtering method where more weight is given to the prediction sample may be used, compared to when the neighboring block is encoded in the inter mode. For example, in the case of that the intra prediction mode is 34, when the neighboring block is encoded in the inter mode, a (1/2, 1/2) filter may be used, and when the neighboring block is encoded in the intra mode, a (4/16, 12/16) filter may be used.

The number of lines to be filtered in the current block may vary depending on the size/shape of the current block (e.g., the coding block or the prediction block). For example, when the size of the current block is equal to or less than 32×32, filtering may be performed on only one line at the block boundary; otherwise, filtering may be performed on multiple lines including the one line at the block boundary.

FIGS. 12 and 13 are based on the case where the 35 intra prediction modes in FIG. 7 are used, but may be equally/similarly applied to the case where the extended intra prediction modes are used.

FIG. 14 shows a range of reference samples for intra prediction according to an embodiment to which the present invention is applied.

Intra prediction of a current block may be performed using a reference sample derived based on a reconstructed sample included in a neighboring block. Here, the reconstructed sample means that encoding/decoding is completed before encoding/decoding the current block. For example, intra prediction for the current block may be performed based on at least one of reference samples P(−1, −1), P(−1, y) (0<=y<=2N−1) and P (x, −1) (0<=x<=2N−1). A prediction sample for the current block may be generated by taking an average of reference samples, or copying a reference sample in a certain direction considering a directionality of an intra prediction mode of the current block.

Intra prediction of the current block may be performed using at least one of a plurality of reference lines. All or a part of lengths of the plurality of reference lines may be determined as the same, or the lengths may be set different from each other.

For example, assuming that the current block has a W×H size, the k-th reference line may include reference samples p(−k, −k), reference samples on the same row as p(−k, −k) (e.g., reference samples from p(−k+1, −k) to p(W+H+2(k−1), −k) or reference samples from p(−k+1, −k) to p(2W+2(k−1), −k)), and reference samples on the same column as p(−k, −k) (e.g., reference samples from p(−k, −k+1) to p(−k,W+H+2(k−1)) or reference samples from p(−k, −k+1) to p(−k,2H+2(k−1))).

FIG. 15 is a diagram exemplifying a plurality of reference sample lines. As in the example shown in FIG. 15, when the first reference line adjacent to the boundary of the current block is referred to as a ‘reference line 0’, the k-th reference line may be configured to be adjacent to the (k−1)-th reference line.

Alternatively, unlike that shown in FIG. 15, it may be configured that all the reference lines have the same number of reference samples.

The intra prediction of the current block may be performed based on reference samples included in at least one reference line selected from the plurality of reference lines. Here, performing an intra prediction by selecting at least one of the plurality of reference line candidates may be referred to as an ‘intra prediction method using extended reference sample (extended reference intra prediction)’ or an ‘extended intra prediction method’. Further, the plurality of reference lines may be referred to as an ‘extended reference lines’.

Whether to perform intra prediction based on the extended reference line may be determined based on information signaled through a bitstream. Here, the information may be a 1-bit flag, but is not limited thereto. Information on whether to perform intra prediction based on the extended reference line may be signaled in a unit of coding tree unit, coding unit, or prediction unit, or may be signaled in a unit of a sequence, a picture, or a slice. That is, whether to perform intra prediction based on the extended reference line may be determined in a unit of a sequence, a picture, a slice, a CTU, a CU, or a PU.

Since intra prediction generates prediction samples using limited reference samples, the generated prediction samples may not reflect features of the original image. That is, since the intra prediction of the current block is performed using only the neighboring samples adjacent to the current block, the features of the original image may not be accurately reflected. For example, in a case where an edge exists in the current block, a new object appears around a boundary of the current block, or the like, the difference between prediction sample and the original image may be large, depending on a position of a prediction sample in the current block.

In this case, the residual value becomes relatively large, and the amount of bits to be encoded/decoded may increase. Accordingly, the present invention proposes a method of refinement of a prediction sample generated through intra prediction. A method for modifying a prediction sample will be described in detail with reference to FIGS. 16 to 24.

FIG. 16 is a flowchart illustrating a method of modifying a prediction sample according to an embodiment of the present invention.

First, an intra prediction image (a prediction block or a prediction sample) for the current block may be obtained, based on the intra prediction mode of the current block S1610. In the following embodiments, a prediction image generated based on an intra prediction mode for a current block is referred to as a first intra prediction image or a first prediction block, and a sample included in the first intra prediction image is referred to as a first prediction sample.

When a first intra prediction image is generated as a result of performing intra prediction, an offset is applied to at least a part of a current block to generate a second intra prediction image (or a second prediction sample) refined from a first intra prediction image (or a first prediction sample) may be generated. For example, refinement of the first intra prediction image based on the offset may be performed on an entire region of the current block, a pre-defined partial region, or a sample at a specific position. An offset used for refining the first intra prediction image to the second intra prediction image may be referred to as an ‘intra refinement offset’.

Whether to apply an offset to at least a partial region of the current block may be determined based on a type of the intra prediction mode of the current block, a directionality of the intra prediction mode, an angle of the intra prediction mode, a size or a shape of the current block (or the prediction block), or the like. For example, it may be configured that, when the intra prediction mode of the current block is a non-directional mode such as a DC mode or a planar mode, an offset is applied to at least a partial region of the current block, whereas when the intra prediction mode of the current block is a directional mode, an offset is not applied to the current block. As another example, it may be configured that, when the intra prediction mode of the current block is an intra prediction mode in a specific direction, an offset is applied to at least a partial region of the current block.

Alternatively, whether to apply an offset to at least a partial region of the current block may be determined by information decoded from a bitstream. For example, a syntax ‘is_predblock_refinement_flag’ indicating whether to refine the first intra prediction image using an offset may be signaled through a bitstream. When a value of ‘is_predblock_refinement_flag’ is 1, refinement of the first intra prediction image may be performed using the offset in the current block, whereas when the value of ‘is_predblock_refinement_flag’ is 0, refinement of the first intra prediction image may not be performed in the current block. When the refinement of the first intra prediction image is not performed, the first intra prediction image may be output as a final prediction result of the current block.

To apply an offset to at least a partial region of the current block, an offset for the current block may be determined S1620.

The offset may be defined for each predetermined unit in the current block. Here, the predetermined unit may mean one sample, a line (e.g., row or column) including a plurality of samples, a sub-block including a plurality of lines, a sub-block of a predetermined size, or the like. For example, after defining an offset for each sample in the first intra prediction image, a second intra prediction image may be generated by adding or subtracting an offset corresponding to each sample to or from each sample.

The offset may be derived from a reference sample of the current block. Specifically, the offset may be derived based on a sum, a difference, an average, a weighted operation value (e.g., a weighted average), an intermediate value, a maximum value, a minimum value, or the like, of a plurality of reference samples (e.g., two, three, or more reference samples).

Reference samples used for deriving the offset may include at least one of a reference sample at a fixed position or a reference sample determined dependently on a position of a prediction sample (i.e., a sample of the first intra prediction image). For example, the offset may be derived based on at least one of a reference sample adjacent to the top left corner of the current block and a reference sample on the same vertical line/horizontal line as the prediction sample to which the offset is to be applied.

Alternatively, the number and/or position of the reference samples used for calculating the offset may be variably determined based on a size, a shape, a direction or an angle of the intra prediction mode of the current block, or the like.

The offset may be derived using a reference sample before a filter (e.g., an AIS filter) is applied thereto. That is, the first intra prediction image may be generated using a reference sample to which filtering has been performed, while the second intra prediction image may be generated using a reference sample before a filter is applied thereto.

The offset may be derived based on a plurality of reference samples included in the same reference line, or may be derived based on reference samples each included in a different reference line.

For example, in the example shown in FIG. 15, the offset may be determined based on a difference value between a value of a reference sample derived from the first reference line and a value of a reference sample derived from one of the second reference line or the fourth reference line.

Equations 7 and 8 show an example of deriving an intra refinement offset. The intra refinement offset h may be determined using one of the equations shown in Equation 7.

h=ref1(i ₁ j ₁)−ref0(i ₀ j ₀)   [Equation 7]

h=ref2(i ₂ j ₂)−ref0(i ₀ j ₀)

h=ref3(i ₃ j ₃)−ref0(i ₀ j ₀)

h=ref4(i₄ j ₄)−ref0(i ₀ j ₀)

Alternatively, a plurality of intra refinement offsets f0, f1, f2, and f3 may be determined as shown in Equation 8. The refinement of the current block may be performed based on any one of the plurality of intra refinement offsets f0 to f3, or may be performed by applying a different intra refinement offset in a unit of a predetermined region.

f0=w ₀*(ref1)(i ₁ j ₁)−ref0(i ₀ j ₀))   [Equation 8]

f1=w ₁*(ref2(i ² j ₂)−ref0(i ₀ j ₀))

f0=w ₂)(ref3(i ₃ j ₃)−ref0(i ₀ j ₀))

f0=w ₃*(ref4(i ₄ j ₄)−ref0(i ₀ j ₀))

In Equations 7 and 8, refn (x=0 to 4) indicates the reference line n. In addition, (i_(n), j_(n)) represents a position of the reference sample included in the reference line n. The value of the reference sample used for deriving the offset may be derived from neighboring reconstructed samples for performing an intra prediction of the current block, or may represent a separate reconstructed sample for calculating the offset. Here, the value of the separate reconstructed sample may be derived from a neighboring sample of the separate reconstructed sample or a sample at a predefined position. The value of the reference sample used for deriving the offset may be a value before an in-loop filter is applied to a neighboring block or a value after the in-loop filter has been applied to the neighboring block.

The offset may be obtained by applying weights to the reference samples. For example, in Equation 8, an offset is obtained by applying a weight w to a differential value between reference samples. In addition, an offset may be obtained through a weighted operation that applies a different weight to each reference sample. Here, a weight applied to each reference sample may be determined based on a distance between a sample at a specific position in the current block and a reference line, a distance between a sample at a specific position and a reference sample, a distance between reference samples, a distance between reference lines, or the like. The sample at the specific position may represent a sample at a predefined position in the current block, or may represent a sample to which an offset is to be applied (hereinafter, referred to as ‘an offset application target sample’, or ‘a refinement target sample’). The sample at the predefined position may include at least one of a sample adjacent to the left boundary of the current block, a sample adjacent to the top boundary of the current block, or a sample adjacent to the top left corner of the current block.

For example, a weight may be determined in proportion to a distance between a sample at a specific position in the current block and a reference sample used for calculating an offset. For example, it is assumed that an offset is derived based on a reference sample at the top left corner of the current block, a reference sample in a vertical direction with respect to the offset application target sample, and a reference sample in a horizontal direction with respect to the offset application target sample. In this case, a weight applied to the reference sample at the top left corner of the current block may be obtained using at least one of an x-axis distance differential value or a y-axis distance differential value between the corresponding reference sample and the offset application target sample. For the sample in the vertical direction with respect to the offset application target sample, it may be obtained based on a y-axis distance differential value between the corresponding reference sample and the offset application target sample, and for the sample in the horizontal direction with respect to the offset application target sample, it may be obtained based on the x-axis distance differential value between the corresponding reference sample and the offset application target sample.

Alternatively, a weight may be determined by adding/subtracting a distance between a sample at a specific position in the current block and a reference sample used for calculating an offset to/from a predefined value.

As another example, a weight may be determined based on a ratio between a value indicating a distance between a sample at a specific position in the current block and a reference sample and a value indicating a size of the current block.

An intra refinement offset may be derived based on a residual sample of a neighboring reconstructed sample (i.e., a reference sample) adjacent to the first intra prediction image. Here, the position of the neighboring reconstructed sample may be specified by at least one of a type, a direction, or an angle of the intra prediction mode used for obtaining the first intra prediction image. For example, the position of the neighboring reconstructed sample may be that of a reference sample positioned on the angular line of the directional intra prediction mode or may be on a line that is orthogonal to the angular line of the intra prediction mode.

Alternatively, the offset value may be signaled in a unit of a slice, a coding unit, or a prediction unit.

When the offset is determined, an offset may be applied to the first intra prediction image to generate a second intra prediction image which is a refinement of the first intra prediction image S1630. For example, the following Equation 9 shows an example of deriving a sample P′ (x, y) of a second intra prediction image from a sample P(x, y) included in a first intra prediction image.

P′(i,j)=P(i,j)+f   [Equation 9]

As shown in Equation 8, the sample P′ (i, j) of the second intra prediction image may be obtained by adding the offset f to the sample P(i, j) of the first intra prediction image. In contrast to Equation 9, the second intra prediction image may be obtained by subtracting the offset from the first intra prediction image. Alternatively, the second intra prediction image may be obtained based on a weighted operation of the first intra prediction image and the offset. Here, the weights applied to the first intra prediction image and the offset may be determined according to the position of the sample to be refined, or the like.

In the case of a directional intra prediction mode, since a prediction sample is generated by copying a reference sample adjacent to the current block, prediction efficiency in a region far from the current block may be deteriorated. That is, when the directional intra prediction mode is used, a residual value in a region relatively far from the boundary of the current block may include a large amount of high-frequency components, which may result in a deterioration in encoding/decoding efficiency.

To solve the above problem, a method for refining a prediction image in a unit of a sub-block may be considered. Here, refining a prediction image in a unit of a sub-block may mean that performing a refinement of a prediction image only in a region corresponding to a predetermined sub-block among an entire region of the current block, or an offset is defined in units of a predetermined sub-block in the current block (i.e., using a different offset for each sub-block). When the prediction image is refined in a unit of a sub-block, the accuracy of prediction in a region relatively far away from the block boundary may be improved. Hereinafter, referring to FIG. 17, a method of refining a prediction image in a unit of a sub-block will be described in detail.

FIG. 17 is a flowchart illustrating a method for refining a prediction image in a unit of a sub-block according to an embodiment of the present invention.

Referring to FIG. 17, for the current block, it may be determined whether to refine (or update) the first intra prediction image in a unit of a sub-block S1700. Whether the refinement of the first intra prediction image is performed in a unit of a sub-block may be determined based on at least one of a size, a shape of the current block, a type, a direction, or an angle of the intra prediction mode. Alternatively, whether to refine the first intra prediction image in a unit of a sub-block may be determined by a flag decoded from a bitstream. For example, a syntax ‘is_sub_block_refinement_flag’ indicating whether to update the first intra prediction image in a unit of a sub-block may be signaled through a bitstream. When a value of ‘is_sub_block_refinement_flag’ is 1, refinement of the first prediction sample using an offset may be performed in a predetermined sub-block in the current block, whereas when the value of ‘is_sub_block_refinement_flag’ is 0, the refinement in units of a sub-block may not be performed.

Whether to refine the first intra prediction image and the refinement unit may be hierarchically determined. For example, only when it is determined that the refinement of the first intra prediction image is performed, it may be determined whether the refinement unit is a sub-block unit.

When it is determined that the refinement of the first intra prediction image is performed in a unit of a sub-block, the intra prediction pattern of the current block may be determined S1710. Through the intra prediction pattern, it may be determined that all or a partial region of the current block to which the offset is applied, the partition type of the current block, whether the offset is applied to the sub-block included in the current block, a size/sign of the offset assigned to each sub-block, or the like.

Any one of a plurality of patterns predefined in the encoder/decoder may be selectively used for an intra prediction pattern of the current block, and for that, an index specifying the intra prediction pattern of the current block may be signaled from a bitstream. As another example, the intra prediction pattern of the current block may be determined based on a partition mode of the prediction unit or coding unit of the current block, a block size/shape, whether being a directional intra prediction mode, an angle of the directional intra prediction mode, an intra prediction pattern of a neighboring block, or the like.

Whether an index indicating an intra prediction pattern of the current block is signaled may be determined by predetermined flag information signaled from a bitstream. For example, when the flag information indicates that the index indicating the intra prediction pattern of the current block is signaled from the bitstream, the intra prediction pattern of the current block may be determined based on the index decoded from the bitstream. Here, the flag information may be signaled in a unit of at least one of a picture, a slice, or a block level.

When the flag information indicates that the index indicating the intra prediction pattern of the current block is not signaled from the bitstream, the intra prediction pattern of the current block may be determined based on a partition mode of the prediction unit or coding unit of the current block, or the like. For example, a partition type in which the current block is partitioned into sub-blocks may have the same type as a partition type in which the coding block is partitioned into prediction units.

When the intra prediction pattern of the current block is determined, an offset may be obtained in a unit of a sub-block S1720. The offset may be signaled in a unit of a slice, a coding unit, or a prediction unit. As another example, the offset may be derived from a neighboring sample (e.g., a reference sample) of the current block. The offset may include at least one of offset size information or offset sign information. Here, the offset size information may be within a range of an integer greater than or equal to zero.

When the offset is determined, for each sub-block, the second intra prediction image may be generated by refining the first intra prediction image S1730. The second intra prediction image may be obtained by applying an offset to the first intra prediction image. For example, the second prediction sample may be obtained by adding or subtracting the offset to or from the first prediction sample. Here, a different offset for each sub-lock may be applied to the first intra prediction image.

FIGS. 18 to 22 are diagrams illustrating intra prediction patterns of a current block according to an embodiment to which the present invention is applied.

For example, in the example shown in FIG. 18, when the index is ‘0’ or ‘1’, the current block is partitioned into top and bottom sub-blocks, an offset may not be set for the top sub-block, and an offset ‘f’ may be set for the bottom sub-block. Accordingly, the first prediction sample (P(i,j)) may be used as it is for the top sub-block, and the second prediction sample (P(i,j)+f or P(i,j)−f) generated by adding or subtracting the offset to or from the first prediction sample may be used for the bottom sub-block. In the present specification, ‘not set’ may mean that no offset is assigned to the corresponding block, or an offset of ‘0’ is assigned to the corresponding block.

When the index is ‘2’ or ‘3’, the current block is partitioned into left and right sub-blocks, an offset may not be set for the left sub-block, and an offset ‘f’ is set for the right sub-block. Accordingly, the first prediction sample (P(i,j)) may be used as it is for the left sub-block, and the second prediction sample (P(i,j)+f or P(i,j)−f) generated by adding or subtracting the offset to or from the first prediction sample may be used for the right sub-block.

Available intra prediction patterns may have limited in its range based on the intra prediction mode of the current block. For example, when the intra prediction mode of the current block is a vertical directional intra prediction mode or a prediction mode having a similar direction to the vertical directional intra prediction mode (for example, when the intra prediction mode index is 22 to 30 among 33 directional prediction modes), only an intra prediction pattern (e.g., index 0 or index 1 in FIG. 18) horizontally partitioning the current block may be applied to the current block.

As another example, when the intra prediction mode of the current block is a horizontal directional intra prediction mode or a prediction mode having a similar direction to the horizontal directional intra prediction mode (for example, when the intra prediction mode index is 6 to 14 among 33 directional prediction modes), only an intra prediction pattern (e.g., index 2 or index 3 in FIG. 18) vertically partitioning the current block may be applied to the current block.

Alternatively, an intra prediction pattern available to the current block may be determined depending on whether the intra prediction mode of the current block is a non-directional mode.

FIG. 18 shows an example that no offset is set for one of sub-blocks included in the current block, and an offset is set for the other. Whether to set an offset for a sub-block may be determined based on information signaled for each sub-block.

Alternatively, whether to set an offset for a sub-block may be determined based on a position of the sub-block, an index for identifying the sub-block in the current block, or the like. For example, with respect to a predetermined boundary of the current block, an offset may not be set for a sub-block adjoining the predetermined boundary, and an offset may be set for a sub-block not adjoining the predetermined boundary. Here, the predetermined boundary may be determined based on a size, a shape, or an intra prediction mode of the current block.

When the predetermined boundary is assumed to be a top boundary of the current block, for the intra prediction pattern corresponding to the index ‘0’ or ‘1’, an offset may not be set for a sub-block adjoining the top boundary of the current block, and an offset may be set for a sub-block not adjoining the top boundary of the current block.

When the predetermined boundary is assumed to be a left boundary of the current block, for the intra prediction pattern corresponding to the index ‘2’ or ‘3’, an offset may not be set for a sub-block adjoining the left boundary of the current block, and an offset may be set for a sub-block not adjoining the left boundary of the current block.

In FIG. 18, it is assumed that no offset is set for one of the sub-blocks included in the current block, and an offset is set for the other. As another example, an offset of a different value may be set for each of the sub-blocks included in the current block.

Referring to FIG. 19, an example in which a different offset is set for each sub-block will be described.

Referring to FIG. 19, when the index is ‘0’ or ‘1’, an offset ‘h’ may be set for the top sub-block in the current block, and an offset ‘f’ may be set for the bottom sub-block in the current block. Accordingly, a second prediction sample (P(i,j)+h or P(i,j)−h) obtained by adding or subtracting the offset ‘h’ to or from the first prediction sample may be generated for the top sub-block, and a second prediction sample (P(i,j)+f or P(i,j)−f) obtained by adding or subtracting the offset ‘f’ to or from the first prediction sample may be generated for the bottom sub-block.

Referring to FIG. 19, when the index is ‘2’ or ‘3’, an offset ‘h’ may be set for the left sub-block in the current block, and an offset ‘f’ may be set for the right sub-block in the current block. Accordingly, a second prediction sample (P(i,j)+h or P(i,j)−h) obtained by adding or subtracting the offset ‘h’ to or from the first prediction sample may be generated for the left sub-block, and a second prediction sample (P(i,j)+f or P(i,j)−f) obtained by adding or subtracting the offset ‘f’ to or from the first prediction sample may be generated for the right sub-block.

In FIGS. 18 and 19, it is described that the current block is partitioned into two sub-blocks having the same size, but the number of sub-blocks included in the current block and/or the size of the sub-block is not limited to examples shown in FIGS. 18 and 19. The number of sub-blocks included in the current block may be three or more, and each sub-block may have a different size.

When a plurality of intra prediction patterns are available, the available intra prediction patterns may be grouped into a plurality of categories. In this case, the intra prediction pattern of the current block may be selected based on a first index for identifying a category and a second index for identifying an intra prediction pattern in the corresponding category.

Referring to FIG. 20, an example in which an intra prediction pattern of a current block is determined based on a first index and a second index will be described.

In the example shown in FIG. 20, twelve intra prediction patterns may be classified into three categories each including four intra prediction patterns. For example, the intra prediction patterns corresponding to the indices 0 to 3 are classified into category 0, the intra prediction patterns corresponding to the indices 4 to 7 are classified into category 1, and the intra prediction patterns corresponding to the indices 8 to 11 are classified into category 2.

A decoder may decode the first index from a bitstream to specify a category including at least one intra prediction pattern. In the example shown in FIG. 20, the first index may specify any one of categories 0, 1, and 2.

When the category is specified based on the first index, the intra prediction pattern of the current block may be determined based on the second index decoded from the bitstream. When category 1 is specified by the first index, the second index may specify any one of the four intra prediction patterns (i.e., index 4 to index 7) included in category 1.

In FIG. 20, it is shown that each category includes the same number of intra prediction patterns, but each category does not necessarily have to include the same number of intra prediction patterns.

The number of available intra prediction patterns or the number of categories may be determined in a unit of a sequence or a slice. In addition, at least one of the number of available intra prediction patterns or the number of categories may be signaled through a sequence header or a slice header.

As another example, the number of available intra prediction patterns and/or the number of categories may be determined based on a size of a prediction unit or a coding unit of the current block. For example, when the size of the current block (e.g., the coding unit of the current block) is 64×64 or more, the intra prediction pattern of the current block may be selected from the six intra prediction patterns shown in FIG. 21. Alternatively, when the size of the current block (e.g., the coding unit of the current block) is smaller than 64×64, the intra prediction pattern of the current block may be selected from the intra prediction patterns shown in FIG. 18, FIG. 19, or FIG. 20.

In FIGS. 18 to 21, it is shown that the sub-blocks included in each intra-prediction pattern are rectangular. As another example, intra prediction patterns in which at least one of the sizes or shapes of sub-blocks are different may be used. For example, FIG. 22 shows an example of an intra prediction pattern having different sizes and shapes of sub-blocks.

An offset for each sub-block (e.g., offset h, f, g, or i for each sub-block shown in FIG. 18 to FIG. 22) may be decoded from a bitstream, or may be derived from a neighboring sample (e.g., a reference sample) adjacent to the current block. For example, as described with reference to FIG. 16, an offset derived from at least one reference sample may be used for a sub-block, or an offset derived from reference samples included in different reference lines (see Equations 7 and 8) may be used for a sub-block.

As another example, the offset of the sub-block may be determined in consideration of a distance from a sample at a specific position in the current block. For example, the offset may be determined in proportion to a value indicating a distance between a sample at a predetermined position in the current block and a sample at a predetermined position in the sub-block.

As another example, the offset of the sub-block may be determined by adding or subtracting a value determined based on a distance between a sample at a predetermined position in the current block and a sample at a predetermined position in the sub-block to or from a predetermined value.

As another example, the offset may be determined based on a ratio between a value indicating a distance between a sample at a predetermined position in the current block and a sample at a predetermined position in the sub-block, and a value indicating a size of the current block.

Here, the sample at a predetermined position in the current block may include a sample adjacent to the left boundary of the current block, a sample located at the top boundary of the current block, a sample adjacent to the top left corner of the current block, or the like.

In a sub-block, a different offset may be applied in a predetermined unit. Specifically, a different offset may be applied for each sample in the sub-block, or a different offset may be applied for each line (row or column) or for each plurality of lines.

FIGS. 23 and 24 illustrate examples in which a different offset is applied in a predetermined unit in a sub-block.

The left diagram of FIG. 23 shows a case in which a different offset is applied in units of a row in a sub-block, and the right diagram shows an example in which a different offset is applied in units of a column in the sub-block. As shown in FIG. 23, a different offset may be used for each line, such as offset f0 in the first line, offset f1 in the second line, offset f2 in the third line, and offset f3 in the fourth line. Here, f0 through f3 may be derived based on a differential value between the reference samples each belonging to a different reference line, as described with reference to Equation 8.

Alternatively, as shown in FIG. 24, the weights to be applied to the offsets may be set differently according to the positions in the sub-block. For example, a different offset may be used for each line, such as offset h in the first line, offset 2h in the second line, offset 3h in the third line, and offset 4h in the fourth line. Here, h may be derived based on a difference value between the reference samples each belonging to a different reference line, as described with reference to Equation 7.

FIG. 25 is a flowchart illustrating processes of obtaining a residual sample according to an embodiment to which the present invention is applied.

First, a residual coefficient of a current block may be obtained S2510. A decoder may obtain a residual coefficient through a coefficient scanning method. For example, the decoder may perform a coefficient scan using a diagonal scan, a jig-zag scan, an up-right scan, a vertical scan, or a horizontal scan, and may obtain residual coefficients in a form of a two-dimensional block.

An inverse quantization may be performed on the residual coefficient of the current block S2520.

It is possible to determine whether to skip an inverse transform on the dequantized residual coefficient of the current block S2530. Specifically, the decoder may determine whether to skip the inverse transform on at least one of a horizontal direction or a vertical direction of the current block. When it is determined to apply the inverse transform on at least one of the horizontal direction or the vertical direction of the current block, a residual sample of the current block may be obtained by inverse transforming the dequantized residual coefficient of the current block S2540. Here, the inverse transform can be performed using at least one of DCT, DST, and KLT.

When the inverse transform is skipped in both the horizontal direction and the vertical direction of the current block, inverse transform is not performed in the horizontal direction and the vertical direction of the current block. In this case, the residual sample of the current block may be obtained by scaling the dequantized residual coefficient with a predetermined value S2550.

Skipping the inverse transform on the horizontal direction means that the inverse transform is not performed on the horizontal direction but the inverse transform is performed on the vertical direction. At this time, scaling may be performed in the horizontal direction.

Skipping the inverse transform on the vertical direction means that the inverse transform is not performed on the vertical direction but the inverse transform is performed on the horizontal direction. At this time, scaling may be performed in the vertical direction.

It may be determined whether or not an inverse transform skip technique may be used for the current block depending on a partition type of the current block. For example, if the current block is generated through a binary tree-based partitioning, the inverse transform skip scheme may be restricted for the current block. Accordingly, when the current block is generated through the binary tree-based partitioning, the residual sample of the current block may be obtained by inverse transforming the current block. In addition, when the current block is generated through binary tree-based partitioning, encoding/decoding of information indicating whether or not the inverse transform is skipped (e.g., transform_skip_flag) may be omitted.

Alternatively, when the current block is generated through binary tree-based partitioning, it is possible to limit the inverse transform skip scheme to at least one of the horizontal direction or the vertical direction. Here, the direction in which the inverse transform skip scheme is limited may be determined based on information decoded from the bitstream, or may be adaptively determined based on at least one of a size of the current block, a shape of the current block, or an intra prediction mode of the current block.

For example, when the current block is a non-square block having a width greater than a height, the inverse transform skip scheme may be allowed only in the vertical direction and restricted in the horizontal direction. That is, when the current block is 2N×N, the inverse transform is performed in the horizontal direction of the current block, and the inverse transform may be selectively performed in the vertical direction.

On the other hand, when the current block is a non-square block having a height greater than a width, the inverse transform skip scheme may be allowed only in the horizontal direction and restricted in the vertical direction. That is, when the current block is N×2N, the inverse transform is performed in the vertical direction of the current block, and the inverse transform may be selectively performed in the horizontal direction.

In contrast to the above example, when the current block is a non-square block having a width greater than a height, the inverse transform skip scheme may be allowed only in the horizontal direction, and when the current block is a non-square block having a height greater than a width, the inverse transform skip scheme may be allowed only in the vertical direction.

Information indicating whether or not to skip the inverse transform with respect to the horizontal direction or information indicating whether to skip the inverse transformation with respect to the vertical direction may be signaled through a bitstream. For example, the information indicating whether or not to skip the inverse transform on the horizontal direction is a 1-bit flag, ‘hor_transform_skip_flag’, and information indicating whether to skip the inverse transform on the vertical direction is a 1-bit flag, ‘ver_transform_skip_flag ’. The encoder may encode at least one of ‘hor_transform_skip_flag’ or ‘ver_transform_skip_flag’ according to the shape of the current block. Further, the decoder may determine whether or not the inverse transform on the horizontal direction or on the vertical direction is skipped by using at least one of “hor_transform_skip_flag” or “ver_transform_skip_flag”.

It may be set to skip the inverse transform for any one direction of the current block depending on a partition type of the current block. For example, if the current block is generated through a binary tree-based partitioning, the inverse transform on the horizontal direction or vertical direction may be skipped. That is, if the current block is generated by binary tree-based partitioning, it may be determined that the inverse transform for the current block is skipped on at least one of a horizontal direction or a vertical direction without encoding/decoding information (e.g., transform_skip_flag, hor_transform_skip_flag, ver_transform_skip_flag) indicating whether or not the inverse transform of the current block is skipped.

Although the above-described embodiments have been described on the basis of a series of steps or flowcharts, they do not limit the time-series order of the invention, and may be performed simultaneously or in different orders as necessary. Further, each of the components (for example, units, modules, etc.) constituting the block diagram in the above-described embodiments may be implemented by a hardware device or software, and a plurality of components. Or a plurality of components may be combined and implemented by a single hardware device or software. The above-described embodiments may be implemented in the form of program instructions that may be executed through various computer components and recorded in a computer-readable recording medium. The computer-readable recording medium may include one of or combination of program commands, data files, data structures, and the like. Examples of computer-readable media include magnetic media such as hard disks, floppy disks and magnetic tape, optical recording media such as CD-ROMs and DVDs, magneto-optical media such as floptical disks, media, and hardware devices specifically configured to store and execute program instructions such as ROM, RAM, flash memory, and the like. The hardware device may be configured to operate as one or more software modules for performing the process according to the present invention, and vice versa.

INDUSTRIAL APPLICABILITY

The present invention may be applied to electronic devices which is able to encode/decode a video. 

1-15. (canceled)
 16. A method of decoding an image with a decoding apparatus, comprising: determining, with the decoding apparatus, a candidate list of a current block in the image, the candidate list including a plurality of candidates, the plurality of candidates being determined based on an intra prediction mode of a neighboring block adjacent to the current block; deriving, with the decoding apparatus, an intra prediction mode of the current block based on the candidate list and index information, the index information specifying one of the plurality of candidates in the candidate list; deriving, with the decoding apparatus, an extended intra prediction mode based on the intra prediction mode of the current block; and performing, with the decoding apparatus, intra-prediction on the current block based on the extended intra prediction mode.
 17. The method of claim 16, wherein the intra prediction mode of the current block corresponds to one of pre-defined intra prediction modes in the decoding apparatus while the extended intra prediction mode does not belong to the pre-defined intra prediction modes in the decoding apparatus.
 18. The method of claim 17, wherein the extended intra prediction mode has a different angle from directional prediction modes of the pre-defined intra prediction modes in the decoding apparatus.
 19. The method of claim 18, wherein whether to derive the extended intra prediction mode is adaptively determined based on a shape of the current block.
 20. The method of claim 19, wherein in response to a case where the shape of the current block is one of square and non-square, the extended intra prediction mode is derived from the intra prediction mode of the current block and the intra-prediction is performed based on reference samples according to the extended intra prediction mode, and wherein in response to a case where the shape of the current block is the other of square and non-square, the intra-prediction is performed based on reference samples according to the intra prediction mode of the current block without deriving the extended intra prediction mode from the intra prediction mode of the current block.
 21. The method of claim 20, wherein the plurality of candidates includes an intra prediction mode having a value obtained by adding or subtracting a constant value to the intra prediction mode of the neighboring block, and wherein the constant value being greater than or equal to
 2. 22. The method of claim 21, wherein a maximum number of the candidates included in the candidate list is equal to 3, 4, 5, or
 6. 23. A method of encoding an image with an encoding apparatus, comprising: determining, with the encoding apparatus, a candidate list of a current block in the image, the candidate list including a plurality of candidates, the plurality of candidates being determined based on an intra prediction mode of a neighboring block adjacent to the current block; determining, with the encoding apparatus, an intra prediction mode of the current block based on the candidate list, index information being encoded to specify the intra prediction mode of the current block among the plurality of candidates in the candidate list; deriving, with the encoding apparatus, an extended intra prediction mode based on the intra prediction mode of the current block; and obtaining, with the encoding apparatus, a residual block of the current block based on an original block of the current block and a prediction block of the current block, the prediction block being obtained based on the extended intra prediction mode; and generating, with the encoding apparatus, a bitstream by encoding the residual block.
 24. A non-transitory computer-readable medium for storing data associated with a video signal, comprising: a data stream stored in the non-transitory computer-readable medium, the data stream comprising index information specifying one of a plurality of candidates included in a candidate list, wherein the index information is used to determine an intra prediction mode of the current block, wherein the plurality of candidates is determined based on an intra prediction mode of a neighboring block adjacent to the current block, and wherein an extended intra prediction mode is derived based on the intra prediction mode of the current block and is used to perform intra-prediction on the current block. 